1. Field of the Invention
This invention relates to a process for isomerizing optically active .alpha.-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)-isovalerate at the asymmetric carbon atom of the 3-phenoxy-benzyl moiety, and to a process for obtaining (S)-.alpha.-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)isovalerate from (R, S)-.alpha.-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)isovalerate.
2. Description of the Prior Art
.alpha.-Cyano-3-phenoxybenzyl 2-(4-chlorophenyl)isovalerate of the formula (I): ##STR1## which has a low toxicity to mammals and a broad spectrum of insecticidal activity (e.g., as disclosed in Japanese patent application (OPI) No. 26425/74, U.S. Pat. No. 3,996,244) contains two asymmetric carbon atoms in the molecule thereof, and, therefore, includes four optical isomers.
.alpha.-Cyano-3-phenoxybenzyl 2-(4-chlorophenyl)isovalerate of the formula (I) above will be referred to hereinafter as "fenvalerate"; (R, S)-.alpha.-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)isovalerate, as "fenvalerate A"; (S)-.alpha.-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)isovalerate, as "fenvalerate A.alpha."; (R)-.alpha.-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)-isovalerate, as "fenvalerate A.beta."; (S)-.alpha.-cyano-3-phenoxybenzyl (R)-2-(4-chlorophenyl)isovalerate, as "fenvalerate B.alpha."; and (R)-.alpha.-cyano-3-phenoxybenzyl (R)-2-(4-chlorophenyl)isovalerate, as "fenvalerate B.beta.".
The relationship of the absolute configuration of the asymmetric carbon atom on the acid moiety to the insecticidal activity of the ester has already been elucidated, and a process for preparing (S)-2-(4-chlorophenyl)isovaleric acid which provides an ester having a greater insecticidal effect is known (Miyakado et al., Agr. Bio. Chem., 39, 267 (1975)). Miyakado et al., supra, describes a method of optical resolution of .alpha.-isopropylphenylacetic acids, and the insecticidal activity of phenylacetic acid esters having an optically active acid moiety.
The relationship of the absolute configuration of the asymmetric carbon atom on the alcohol moiety to the insecticidal activity of the ester is disclosed in Japanese patent application No. 135013/76 which also describes a method for preparing ester having an optically active alcohol moiety. Japanese patent application No. 99071/76 (corresponding to U.S. Ser. No. 825,507, filed Aug. 17, 1977) discloses a process for producing a fenvalerate isomer having an optically active alcohol moiety by chromatographing fenvalerate A on a silica gel column to separate fenvalerate A into fenvalerate A.alpha. and fenvalerate A.beta.. The method disclosed in Japanese patent application No. 135013/76 proceeds through optically active 3-phenoxymandelic acid obtained by optical resolution of 3-phenoxymandelic acid. Although these methods are suitable on a laboratory scale, they are far from being industrially feasible.
Fenvalerate A.alpha. is an excellent compound having an insecticidal activity of about 4 times greater than the racemic fenvalerate (e.g., as disclosed in Japanese patent application No. 99071/76 (corresponding to U.S. Ser. No. 825,507, filed Aug. 17, 1977)).